High-speed digital data transfers via the so called “Internet” have become ubiquitous in modem society. At the same time, the world has experienced an explosion in wireless communications technology. In well-developed countries like the United States, wireless communications, particularly cellular telephone services, have proliferated as an adjunct to the wired communication network backbone. In less developed countries, wireless communication service is being developed as a primary communications medium. A need has arisen to provide digital data wireless service at ever increasing effective data rates.
Wireless radio telecommunications systems enable many mobile stations or subscribers to connect to land-based wire-line telephone systems and/or digital Internet service providers enabling access to the World Wide Web digital information backbone. Conventional wireless air-interfaces include frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA), and improvements therein.
Transfer of digital data packets differs from the transfer of digital voice information. Full duplex (simultaneous two-way) voice communication patterns imply that the data transferred between the base station and a particular mobile are real-time and substantially equal in bandwidth. It has been noted that a total delay of 200 msec (about 2 Kbit of digital data for most speech vocoders) represents intolerable latency within a voice channel. On the other hand, for high speed data packet transfers, mobile stations appear to be far more tolerant of data transfer latencies or delays, with latencies of up to 10 seconds being encountered in current wireless data systems. While such delays appear to be tolerated by the mobile station, the delays, attributable to relatively low effective data transfer rates, are nonetheless undesirable.
Adaptive antenna array technologies used in attempting to optimize data throughput are known. Examples of spatial diversity multiple access methods employing adaptive antenna arrays are described in U.S. Pat. Nos. 5,471,647 and 5,634,199 to Gerlach et al.; an article by M. C. Wells, entitled: “Increasing the capacity of GSM cellular radio using adaptive antennas”, IKE (UK) Proc. on Comm. Vol. 143, No. 5, October 1996, pp. 304–310; and an article by S. Anderson, B. Hagerman, H. Dam, U. Forssen, J. Karlsson, F. Kronestedt, S. Mazur and K. Molinar, entitled: “Adaptive Antennas for GSM and TDMA Systems”, IEEE Personal Communications, June 1999, pp. 74–86, all of which are incorporated by reference.
One proposed solution, known as “CDMA/HDR”, uses known techniques to measure channel data transfer rate, to carry out channel control, and to mitigate and suppress channel interference. One approach of this type is more particularly described in a paper by Paul Bender, Peter Black, Matthew Grob, Robert Padovani, Nagabhushana Sindhushayana and Andrew Viterbi, entitled: “CDMA/HDR: A Bandwidth Efficient High Speed Wireless Data Service for Nomadic Users”, published by Qualcomm Corporation. The disclosure of this article is incorporated herein in its entirety.
Another proposed solution is TIA/EIA interim standard, TIA/EIA/IS-2000-2 published by Telecommunications Industry Association in August, 1999. TIA/EIA/IS-2000-2 is the physical layer standard for cdma2000 spread spectrum systems, also part of the cdma2000 standard series. cdma2000 spread spectrum systems support both voice and data communications.
In cdma2000 systems and many of the new generation personal communications systems, a dedicated pilot is introduced in the reverse link. The reverse link dedicated pilot signal is an unmodulated spread spectrum signal used to assist the base station in detecting a mobile station transmission. The reverse link dedicated pilot signal is integrated at the base station and used for at least two purposes: reverse link power control and coherent demodulation of the reverse link signals.
A reverse link power control mechanism is used to ensure that all the received dedicated pilots at the base station have the same signal to interference-plus noise ratios (SINRs). Even though the received dedicated pilots have the same SINRs, the transmitting powers of these dedicated pilots from different mobile stations can be different. The dedicated pilot transmitting powers depend on the required pilot SINR at the base station and the radio propagation channels.
A coherent demodulation mechanism is used to increase base station receiver sensitivity. It is common knowledge that in most cases coherent demodulation provides approximately 3 dB better receiver sensitivity than non-coherent demodulation. Base station receiver sensitivity is further increased by use of maximum ratio combining (MRC) methods. FIG. 1A shows a typical base station that implements coherent demodulation and maximum ratio combining mechanisms. Signals received from a plurality of receiving elements 10 are transmitted to a corresponding one of analog receivers 12. After processing, the analog signals are converted to digital signals by analog-to-digital converters (ADCs) 14. Coherent demodulators 18 then process signals from ADCs 14 and pilot signal integration circuitry 16. The outputs of demodulators 18 are then transmitted to maximum ratio combiner 19 to output a selected signal.
Although a maximum combining mechanism with multiple antenna elements can increase the base station receiver sensitivity, it does not provide interference cancellation. (In high capacity personal communications systems, the data throughput is usually limited by interference, which is composed of inter-cell and intra-cell interference.) On the other hand, the standard interference cancellation algorithms, for example, a direct matrix inversion algorithm, require a large amount of numerical computations, making the systems either impractical or expensive to build. Thus, a hitherto unsolved need has arisen for a more efficient and practical method for sending data to a base station by a plurality of mobile stations being served without causing unacceptable interference to each other.
A need therefore exists in the art for systems and methods which optimize data throughput without explicit implementation of interference cancellation algorithms of the prior art.